Refrigeration unit with air humidity monitoring

ABSTRACT

A refrigeration appliance includes a storage chamber, an evaporator that cools the storage chamber, and a processing unit configured to assign values for the air humidity in the storage chamber and an evaporation temperature of the evaporator to one another, at a given temperature of the storage chamber, under the assumption that the absolute water vapor content of the air of the storage chamber is the same as that of water-vapor-saturated air at the evaporation temperature. This assignment makes it possible on one hand to estimate the relative air humidity in the storage chamber on the basis of the temperatures, and on the other hand to control the air output temperature, at constant temperature in the storage chamber, in order to influence the air humidity therein.

The present invention relates to a refrigeration appliance, inparticular a household refrigeration appliance.

The relative air humidity present in a storage chamber of arefrigeration appliance is of major significance for the shelf life offood therein. A high level of air humidity is desirable in particularfor storing fresh vegetables. In refrigeration appliances in which airis circulated between the storage chamber and a heat exchanger,typically in what are referred to as no-frost refrigeration appliances,such a high level of air humidity is difficult to maintain, as when theair is cooled to below its dew point on a surface of the heat exchanger,it loses moisture there, said moisture having to be discharged from therefrigeration appliance and eliminated. This water loss causes thevegetables to wilt rapidly.

In order to be able to store vegetables for longer, vegetablecompartments have been developed, which comprise a container that can beclosed with a lid. When the container is sealed, the moisture given offby vegetables contained therein cannot escape and at the same time thecontainer can be cooled by cold air circulating over its outer surfaces.A limited exchange of air can be permitted between the containerinterior and its surroundings through an adjustable opening in thecontainer, so that different air humidity values can be set in thecontainer based on the size of the opening. It is difficult to measurethis air humidity and display it on the outside of the refrigerationappliance so that it is visible to a user, as the air humidity sensorwould have to be positioned in the container and its output signal wouldhave to be transmitted from the container, which can be moved about inthe refrigeration appliance, to a display instrument of therefrigeration appliance, without restricting the freedom of movement ofthe container.

Air humidity is therefore generally not measured and the user generallydoes not know the actual level of air humidity in the container. Saiduser can therefore essentially only use his/her own observations todetermine the degree of opening of the container that is appropriate forspecific chilled goods. Such observations are however complicated by thefact that the air humidity in the interior of the container is not onlya function of the size of the opening but is also influenced,indirectly, by ambient conditions by way of the operating time of theevaporator. This makes it difficult for the user to identify arelationship between the degree of opening of the container and theshelf life of the chilled goods.

It is the object of the invention to create a refrigeration applianceand an operating method for a refrigeration appliance, which allow theuser to monitor the air humidity in the refrigeration appliance moreeffectively.

The object is achieved on the one hand by a refrigeration appliance witha storage chamber and an evaporator that cools the storage chamber,wherein a processing unit is designed to assign values for air humidityin the storage chamber and an evaporation temperature of the evaporatorto one another at a defined temperature of the storage chamber, underthe assumption that the absolute water vapor content of the air in thestorage chamber is the same as that of water vapor-saturated air, i.e.air with 100% relative humidity, at the evaporation temperature.

The assignment can be made in different directions and serve differentpurposes. In a first embodiment sensors are provided to measure thetemperature of the storage chamber and the evaporation temperature andthe processing unit is designed to adopt a value of the evaporationtemperature as defined on assignment, to estimate the corresponding airhumidity at the temperature of the storage chamber for said value and todisplay the air humidity thus estimated on a display instrument. Thisallows a user to reach a quantitative conclusion about the air humidityin the storage chamber without an air humidity sensor having to beprovided directly therein, which is problematic in particular when thestorage chamber is configured as a pull-out box as is usually the casefor vegetable compartments.

Even more important than knowledge of the air humidity in the storagechamber is the ability of a user to set the air humidity in the storagechamber specifically. Therefore in a second embodiment the assignment ismade in the reverse direction. To this end an operating element isprovided to set the air humidity in the storage chamber as a set pointvariable and the processing unit is connected to the operating elementand designed to estimate a target value of the evaporation temperaturefor a defined set point value of the air humidity as set at theoperating element on assignment and to align the real evaporationtemperature of the evaporator with the target value. When the air in thestorage chamber in contact with the evaporator is cooled down to theevaporation temperature, only the amount of water vapor that correspondsto 100% relative air humidity at the evaporation temperature can beretained therein. By selecting an appropriate evaporation temperature itis possible to set a desired relative air humidity for the same airafter heating to the temperature of the storage chamber.

In order to allow control of the evaporation temperature, the evaporatorcan be part of a refrigerant circuit, in which the throughput of acompressor can be controlled in order to vary the pressure in theevaporator.

Alternatively or additionally the evaporation temperature can also beinfluenced by controlling the opening cross section of at least onethrottle valve connected in series upstream or downstream of theevaporator.

When the temperature of the storage chamber is constant, if a high levelof air humidity is to be maintained in the storage chamber, theevaporation temperature must be set higher than for a low air humidityvalue.

In particular but not only in the case of a no-frost model refrigerationappliance, with an evaporator chamber separated from the storagechamber, a fan with controllable throughput can be provided to circulateair between the evaporator and the storage chamber.

The faster the fan operates, the smaller the temperature differencebetween upstream and downstream sides of the evaporator. When itoperates slowly, the heat input at the downstream end of the evaporatoris small and a low evaporation temperature can be established there,which removes a large amount of moisture from the air flowing through;when the fan operates quickly, a lot of heat reaches the downstreamregion of the evaporator so a higher evaporation temperature resultswith a constant refrigerant throughput. Therefore the control circuitcan predefine a higher fan speed to maintain a high level of relativeair humidity in the storage chamber at the defined temperature than whena low level of air humidity is set.

The object of the invention is also a method for estimating the relativeair humidity in a refrigeration appliance, in particular a refrigerationappliance as described above, with the steps

-   -   a) measuring the temperature of the storage chamber,    -   b) measuring an evaporation temperature of the evaporator,    -   c) calculating the relative humidity of air at the temperature        of the storage chamber, the absolute moisture content of which        is the same as that of water vapor-saturated air at the        evaporation temperature.

The relative humidity thus calculated can be displayed on a displayinstrument of the refrigeration appliance as an estimated value for therelative air humidity in the storage chamber of the refrigerationappliance.

A further object of the invention is a method for operating arefrigeration appliance, in particular a refrigeration appliance asdescribed above, with the steps

-   a′) determining a set point temperature of a storage chamber,-   b′) determining a set point air humidity of the storage chamber,-   c′) setting an evaporation temperature of the evaporator such that    the absolute moisture content of water vapor-saturated air at the    evaporation temperature is the same as that of air with the set    point air humidity at the set point temperature.

Further features and advantages of the invention will emerge from thedescription of exemplary embodiments which follows with reference to theaccompanying figures, in which:

FIG. 1 shows a block diagram of an inventive refrigeration appliance;

FIG. 2 shows a schematic cross section through a part of the housing ofthe refrigeration appliance; and

FIG. 3 shows a diagram of the relationship between evaporationtemperature and relative air humidity in the vegetable compartment ofthe refrigeration appliance

FIG. 1 shows a block diagram of a household refrigeration appliance witha number of storage chambers 1, 2, 3, each cooled by an evaporator 4, 5or 6. The evaporators 4, 5, 6 are connected to one another in series ina refrigerant circuit. FIG. 1 shows three storage chambers andevaporators but the principle of the invention set out in the followingcan also be applied to refrigeration appliances with any number ofstorage chambers, including just one.

A speed-regulated compressor 7 is connected to a suction connection ofthe last, 6, of the evaporators connected in series. A control circuit 8controls the speed of the compressor 7 based on temperatures measured inthe storage chambers 1, 2, 3 by means of temperature sensors 9 to 11 toa value, at which the output of the compressor 7 suffices to meet thecooling requirements of the storage chambers 1 to 3. In the simplestinstance such regulation can be based on incrementing the compressorspeed when the temperature in one of the storage chambers 1, 2, 3 movesout of a set point interval in an upward direction and decrementing saidspeed when it moves out of the interval in a downward direction.

The refrigerant, which has been compressed in the compressor 7 andadiabatically heated in the process, outputs its heat to thesurroundings by way of a condenser 12, passing back from there to theevaporators 4, 5, 6.

Connected in series upstream of each evaporator is a throttle valve 13,14 or 15, which can be controlled by the control circuit 8. The throttlevalves 13, 14, 15, which are connected one after the other, form a flowresistance, which determines the mass throughput of the refrigerantcircuit. The way in which the flow resistance is distributed to theindividual throttle valves 13, 14, 15 is variable, in other words whenone of the throttle valves is narrowed, another can be widened, so thatthe mass throughput remains the same. It is thus possible for example toraise the pressure and therefore the evaporation temperature in theevaporator 4 by reducing the flow resistance of the throttle valve 13and at the same time increasing the flow resistance of the throttlevalve 14, without this affecting pressure and temperature in thedownstream evaporators 5, 6.

According to one embodiment of the invention the relationship betweenopening cross sections to be set at the valves 13, 14 in order to varythe pressure in the evaporator 4 and at the same time keep the pressuresin the evaporators 5, 6 constant, is preprogrammed in the controlcircuit 8. In a second embodiment after the throttle valve 13 has beenadjusted, the throttle valve 14 is set in such a manner that anoperating point of the compressor 7 characterized by speed and electricpower consumption is restored, thereby ensuring that the mass throughputof the refrigerant circuit is kept constant.

FIG. 2 shows a schematic cross section through a part of the housing ofthe refrigeration appliance with the storage chamber 1. The storagechamber 1 is shown here as the lowest storage chamber of the housing butit can also be in other positions. The position of the storage chamber 1in the lower region of the housing close to a base allows a pull-out box16 to facilitate access to the chilled goods stored therein. In contrastto conventional refrigeration appliances the pull-out box 16 is notclosed and does not need to be, in order to protect its contents againstdrying out. In order to protect the chilled goods from contact withpooled water, a grid or mesh can advantageously be the substrate for thechilled goods instead of the pull-out box 16.

The evaporator 4 is a no-frost evaporator accommodated in an evaporatorchamber 18 separated from the remainder of the storage chamber 1 by awall 17. It also comprises a fan 19, which can be operated at variablespeed by the control circuit 8 in order to suck air through theevaporator 4 and guide the air thus cooled back into the storage chamber1 and around the pull-out box 16 by way of a rear wall channel 20.

The temperature sensor 9 is positioned at a point of the storage chamber1 where it is protected from a direct flow of air exiting from the rearwall channel 20, in this instance for example in a side wall of therefrigeration appliance housing, opposite a side of the pull-out box 16.

A further temperature sensor 21 is provided in the evaporator chamber 18and can be positioned directly in the evaporator 4 itself; FIG. 2 showsit downstream of the evaporator 4 at a point where it is directlyexposed to the flow of air cooled to the evaporation temperature in theevaporator 4.

In normal operating conditions the evaporator is always several ° C.colder than the air in the storage chamber 1. When this air cools tobelow its dew point as it passes through the evaporator 4, some of themoisture carried with it condenses on the evaporator 4 and the airexiting from the evaporator 4 has a relative humidity of 100%. When thisair passes back into the storage chamber 1 and heats up to thetemperature prevailing there, its relative humidity drops accordingly,according to the formula

$\begin{matrix}{{r = {{{100 \cdot 10}\frac{a\; {TD}}{b + {TD}}} - \frac{a\; T}{b + T}}},} & (1)\end{matrix}$

where T is the air temperature in the storage chamber 1 measured by theair sensor 9 and TD is the evaporation temperature measured by thetemperature sensor 21 and the constants a, b can have different valuesdepending on the type of phase transition taking place at the evaporator4. At an evaporation temperature over 0° C., for a phase transition fromvapor to water, a=7.5, b=237.3, at an evaporation temperature below 0°C. and for a phase transition from vapor to ice the values are a=9.5 andb=265.5. An evaporation temperature >0° C. is sufficient for the storagechamber 1 to be used as a vegetable compartment.

A display instrument 22, on which the control circuit 8 outputs theestimated value for air humidity in the storage chamber 1 as calculatedaccording to the above formula (1), can be arranged, as shown in thefigure, on the outside of the housing of the appliance or it can bemounted inside adjacent to the storage chamber 1 at a point where it isonly visible when the door 23 opens.

Clearly the above formula is not simply suitable for estimating the airhumidity in the storage chamber 1 with knowledge of the values measuredby the temperature sensors 9, 21; conversely for a defined set pointtemperature of the storage chamber 1 and a set point air humidity of thestorage chamber 1 set at an operating element 24 by a user a temperaturecan be determined, which, if measured by the sensor 21, would give thedesired air humidity in the storage chamber 1. FIG. 3 shows a schematicdiagram of the relationship between evaporation temperature TD, storagechamber temperature T and relative humidity r. If the evaporationtemperature TD were equal to the compartment temperature T, there wouldbe no condensation at the evaporator 4, no moisture would therefore beextracted from the storage chamber 1 and the air humidity r could reacha value of 100%. As the evaporator 4 has to be colder than the chamber 1in order to cool it, in practice TD is smaller than T and an airhumidity r of below 100% is reached. In practice the evaporationtemperature TD is always a few ° C. below the compartment temperature T,for example at a set point temperature of the storage chamber of +3° C.,the evaporator 4 can be regulated to a temperature TD of +1° C., meaningthat the extraction of moisture from the air at the evaporator 4 isminimal and the relative air humidity r, which is established in thestorage chamber 1, is just below 100%. With a lower evaporationtemperature TD of for example −5° C. much more moisture is eliminated atthe evaporator 4 so that a lower relative air humidity r is establishedin the storage chamber 1. The control circuit 8 is thus able, based onthe values measured by the temperature sensors 9, 21, to establish arelative air humidity in the storage chamber 1 as predefined by the userat the operating element 24. No lid is therefore required on thepull-out box 16 to keep the air humidity in the interior of the pull-outbox high. It is therefore possible with the inventive refrigerationappliance to keep chilled goods that are sensitive to evaporation freshfor a long time without a lid impeding access to the chilled goods.

REFERENCE CHARACTERS

-   1 Storage chamber-   2 Storage chamber-   3 Storage chamber-   4 Evaporator-   5 Evaporator-   6 Evaporator-   7 Compressor-   8 Control circuit-   9 Temperature sensor-   10 Temperature sensor-   11 Temperature sensor-   12 Condenser-   13 Throttle valve-   14 Throttle valve-   15 Throttle valve-   16 Pull-out box-   17 Wall-   18 Evaporator chamber-   19 Fan-   20 Rear wall channel-   21 Temperature sensor-   22 Display instrument-   23 Door-   24 Operating element

1-10. (canceled)
 11. A refrigeration appliance, comprising: a storagechamber; an evaporator for cooling said storage chamber; and aprocessing unit configured to assign values for air humidity in saidstorage chamber and an evaporation temperature of said evaporator to oneanother at a defined temperature of said storage chamber, under anassumption that an absolute water vapor content of air in said storagechamber equals an absolute water vapor content of water vapor-saturatedair at the evaporation temperature.
 12. The refrigeration applianceaccording to claim 11, which further comprises: sensors for measuringthe temperature of said storage chamber and the evaporation temperature;said processing unit being configured to estimate the air humidity byassignment to a defined value of the evaporation temperature; and adisplay instrument connected to said processing unit for displaying anestimated air humidity.
 13. The refrigeration appliance according toclaim 11, which further comprises: an operating element for setting theair humidity in said storage chamber as a set point variable; saidprocessing unit being connected to said operating element and beingconfigured to estimate a target value of the evaporation temperature byassignment to a defined value of the air humidity and to align a realevaporation temperature of said evaporator with the target value. 14.The refrigeration appliance according to claim 13, which furthercomprises a compressor having a mass throughput being used by saidprocessing unit to control the evaporation temperature of saidevaporator.
 15. The refrigeration appliance according to claim 13, whichfurther comprises at least one throttle valve connected in series withsaid evaporator, said at least one throttle valve having an openingcross section being used by said processing unit to control theevaporation temperature of said evaporator.
 16. The refrigerationappliance according to claim 11, wherein the evaporation temperature ishigher for a high defined value than for a low defined value of the airhumidity, at a defined temperature of said storage chamber.
 17. Therefrigeration appliance according to claim 11, which further comprises afan with a controllable throughput for circulating air between saidevaporator and said storage chamber.
 18. The refrigeration applianceaccording to claim 17, wherein the refrigeration appliance is a no-frostappliance.
 19. The refrigeration appliance according to claim 17,wherein said fan has a throughput being higher for a high defined valueof air humidity than for a low defined value of air humidity, at adefined temperature of said storage chamber.
 20. A method for estimatinga relative air humidity in a refrigeration appliance or a householdrefrigerator having a storage chamber and an evaporator for cooling thestorage chamber, the method comprising the following steps: a) measuringa temperature of the storage chamber; b) measuring an evaporationtemperature of the evaporator; and c) calculating a relative humidity ofair at the temperature of the storage chamber, while an absolutemoisture content of the air in the storage chamber is equal to anabsolute moisture content of water vapor-saturated air at theevaporation temperature.
 21. A method for operating a refrigerationappliance or a household refrigerator having a storage chamber and anevaporator for cooling the storage chamber, the method comprising thefollowing steps: a′) determining a set point temperature of the storagechamber; b′) determining a set point air humidity of the storagechamber; and c′) setting an evaporation temperature of the evaporator tocause an absolute moisture content of water vapor-saturated air at anevaporation temperature to be equal to an absolute moisture content ofair having the set point air humidity at the set point temperature.